This invention relates to the preparation of preceramic polycarbosilane derivatives which are prepared by the halogenation of preceramic polycarbosilanes which contain Si-H groups. These halogenated polycarbosilanes can be further treated to yield other polycarbosilane materials with novel functionalities. The derivatized polycarbosilane materials of this invention can be pyrolyzed at elevated temperatures to yield ceramic materials.
Preceramic polycarbosilanes which can be converted to ceramic materials are known in the art. These polycarbosilanes generally contain units of the general formula ##STR1## as well as Si-H functionality which may be in the form of ##STR2## Attempts to incorporate different functional groups in polycarbosilanes via the Si-H units or otherwise have generally been unsuccessful. For example, Seyferth in The Final Scientific Report for Grant No. AF-AFOSR-83-0003 entitled "Organosilicon Compounds and Organosilicon Polymer Intermediates" reported that an attempt to crosslink a liquid Si-H containing polysilane by "chlorination of the Si-H bonds followed by reaction with ammonia" was unsuccessful. Details of this attempted chlorination were not given.
Based on the prior art, the Si-H groups in polycarbosilanes appear to be generally unreactive. Preceramic fibers prepared from polycarbosilanes of the prior art generally require curing by oxygen at temperatures of about 170.degree. C. or more to render them infusible prior to pyrolsis. See, for example, Yajima et al., J. Mat. Sci., 13, 2569 (1978), Yajima, Bull Amer. Ceram. Soc., 62, 893 (1983), Hasegawa et al., J. Mat. Sci., 18, 3633 (1983), and Ichikawa et al., J. Mat. Sci. Lett., 6, 420 (1987). In the Bull. Amer. Ceram. Soc. article Yajima prepared ceramic fibers from polycarbosilanes which had been rendered infusible prior to pyrolysis by heating in air at 190.degree. C. The resulting fibers contained 15.5 weight percent oxygen most of which was thought to be incorporated into the fiber during the curing step. In the J. Mat. Sci. Lett. article, Ichikawa prepared ceramic fibers from polycarbosilanes which had been rendered infusible prior to pyrolysis by heating in air at 170.degree.-200.degree. C. for 0.50-2.0 hours. The cured fibers contained about 9-15 weight percent oxygen whereas the resulting ceramic fibers contained about 9-18 weight percent oxygen. Ichikawa also reported that the polycarbosilane fibers must contain at least about nine weight percent oxygen to survive the pyrolysis step without melting or fusing.
Other polycarbosilanes are known in the art. Verbeek et al. in German Application Publication No. 2,236,078, which is hereby incorporated by reference, prepared ceramic materials by firing a polycarbosilane prepared by the pyrolysis of monosilanes at elevated temperatures in an inert atmosphere.
Yajima et al. in U.S. Pat. Nos. 4,052,430 (Oct. 4, 1977) and 4,100,233 (July 11, 1978), which are both hereby incorporated be reference, prepared ceramic fibers by the pyrolysis of polycarbosilanes in an inert atmosphere or in a vacuum at an elevated temperature. The preceramic fibers could be rendered infusible prior to pyrolysis by treatment with an oxidizing gas (including air, ozone, oxygen, chlorine, and bromine) at a temperature of 50.degree. to 400.degree. C. whereby an oxide layer was formed on the fiber surface. The polycarbosilanes were prepared by thermally decomposing and polycondensing polysilanes.
Yajima et al. in U.S. Pat. Nos. 4,220,600 (Sept. 2, 1980) and 4,283,376 (Aug. 11, 1981), which are both hereby incorporated be reference, prepared ceramic materials by the pyrolysis of polycarbosilanes partly containing siloxane bonds at an elevated temperature under an inert atmosphere or a vacuum. These polycarbosilanes were prepared by heating polysilanes in the presence of about 0.01 to 15 weight percent of a polyborosiloxane in an inert atmosphere.
Iwai et al. in U.S. Pat. No. 4,377,677 (Mar. 22, 1983), which is hereby incorporated by reference, also produced ceramic materials by the pyrolysis of polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The polycarbosilanes of Iwai were prepared by heating a polysilane at 50.degree.-600.degree. C. in an inert gas, distilling out a low molecular weight polycarbosilane fraction and then polymerizing the distilled fraction at 250.degree. to 500.degree. C. in an inert atmosphere.
Schilling et al. in U.S. Pat. No. 4,414,403 (Nov. 8, 1983), which is hereby incorporated by reference, produced ceramic material by the pyrolysis of branched polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The branched polycarbosilanes were prepared by reacting monosilanes with an active metal in an inert solvent at elevated temperatures where at least some of the monosilanes contain vinyl groups or halomethyl groups capable of forming branching during the polymerization. The branched polycarbosilanes produced were not described as containing either vinyl groups or halomethyl groups.
More recently, Seyferth in U.S. Pat. No. 4,650,837 (Mar. 17, 1987) reported the reaction of a polycarbosilane with an alkali metal silylamide. The resulting polycarbosilane gave higher ceramic yields than did the original polycarbosilane.
What has been discovered are new polycarbosilanes which contain halogen or other functional groups. These new polycarbosilanes may be produced by a simple and convenient method. The new polycarbosilane materials may be converted into ceramic materials by pyrolysis at elevated temperatures.